Group III nitride semiconductor materials have recently garnered attention as semiconductor materials used in light-emitting devices for emitting short-wavelength light. A group III nitride semiconductor is generally formed in layers by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor-phase epitaxy (HVPE), or the like using sapphire crystals and other types of oxide crystals, as well as group III-V compound semiconductor crystals, as substrates.
It has been difficult to form a p-type semiconductor that has an adequate carrier concentration from a group III nitride semiconductor. However, it has been discovered that a p-type semiconductor that has an adequate carrier concentration can be obtained by a method whereby gallium nitride (GaN) doped with Mg is irradiated with a low-speed electron beam (see JP (Kokai) No. 2-257679); a method whereby gallium nitride doped in the same manner with Mg is heat-treated in a hydrogen-free atmosphere (see JP (Kokai) No. 5-183189); and by other methods.
The mechanism whereby an adequate carrier concentration is obtained is said to involve activating a hydrogen-passivated p-type dopant in a semiconductor by dehydrogenating the dopant by the abovementioned methods. Metal organic chemical vapor deposition (MOCVD) is generally used for growing a group III nitride semiconductor having good crystallinity. However, the hydrogen gas used as the carrier gas for transporting the starting material compound onto the substrate, or the hydrogen molecules or the radical or atomic hydrogen generated by the decomposition of the ammonia (NH3) used as the group V starting material, is present in a high concentration in the growth apparatus for growing crystals in the MOCVD method. This hydrogen is incorporated into the crystal during growth of the crystal layer of the group III nitride semiconductor and forms a bond during cooling from the growth temperature of the crystal layer with the p-type dopant used to dope the crystal. The p-type dopant that is passivated by hydrogen in this manner is not active and does not create a positive hole. However, by irradiating this crystal layer with an electron beam and performing a heat treatment thereon, the bond between the p-type dopant and hydrogen in the crystal is broken, and it becomes possible to expel the hydrogen from the crystal and to activate the p-type dopant.
However, the step for removing the group III nitride semiconductor from the growth apparatus and performing post-processing is complex in the abovementioned method, and the cost is high. This method also has drawbacks in that the crystallinity is reduced by the heat treatment because nitrogen is desorbed at the same time as dehydrogenation is performed by the heat treatment, and that the luminous intensity is low in the light-emitting device created using this method.
It is also reported that an adequate carrier concentration is obtained by substituting the H2 gas and NH3 gas with an inert gas and performing cooling when the group III nitride semiconductor is cooled to room temperature after growth (see JP (Kokai) No. 8-125222). However, nitrogen substitution is performed in a vacuum in this method, nitrogen is desorbed from the crystal, perhaps due to the vacuum state, and crystallinity is reduced. Substituting with an inert gas without going through the vacuum state has also been proposed (see JP (Kokai) No. 9-129929). However, substitution with an inert gas is performed at 1100° C. in this method, which is the growth temperature of the nitride semiconductor, so nitrogen is desorbed from the crystal, and crystallinity declines. Two to three hours are also required to reach room temperature after the substitution with an inert gas in this method.